Field of the Invention
The present invention relates to an image processing technique for embedding additional information in an image.
Description of the Related Art
In recent years, a technique (referred to as a digital watermark technique) of multiplexing additional information, such as an author name or the existence or absence of usage permission, in image information, such as a photograph, picture, or the like, in a way that is hard to distinguish the additional information visually, has been standardized. As another application field, in conjunction with improvements in image quality of image outputting apparatuses, such as a copying machine or a printer, and with the objective of preventing improper forgery of banknotes, stamps, securities, or the like, there is a technique of embedding additional information, in an image output to paper, to specify an output device and a body number therefor from the image.
Meanwhile, photo books created by laying-out digital photographs in a page and book-binding a plurality of pages have spread. For example, when a bookbinding method, in which a gutter portion becomes flat when the photo book is opened, is selected, a flexible back 101 (or hollow back) is used as a type of back for the bookbinding, as shown on FIG. 1A. For the photo book created with this bookbinding method, because a portion for the back of the photo book is made from a soft material, it is possible to open the pages to a full double-page spread, and a gutter portion 103 becomes flat when the photo book is opened. Meanwhile, if a more common bookbinding method is selected, a tight back 102, as shown in FIG. 1B, is used. For a photo book created with this bookbinding method, because the back portion of the photo book is made from a hard material, how the pages open is restricted, and a valley fold is formed in the gutter portion 103 when the photo book is opened.
There is a method that uses an image capturing device (hereafter, a camera) as a method of reading additional information printed when a photo book, in which additional information is embedded in a photograph used in the photo book, by using the digital watermark technique, is created. When using a bookbinding method for which the gutter portion 103 becomes flat without forming a valley fold, as with the flexible back 101, it is possible to read the additional information from an image captured by the camera. However, when a bookbinding method for which a valley fold is formed in the gutter portion 103, as in the tight back 102, is used, a capturing angle for an imaged surface of the photo book in relation to the imaging direction of the camera becomes non-perpendicular with respect to the camera as the gutter portion 103 is approached. A common digital watermark technique is to multiplex information by embedding additional information having a high-frequency component into an image, and combining specific patterns thereof. If additional information embedded in a photograph is imaged in a state in which the capturing angle is not perpendicular, the high-frequency component of the additional information becomes an even higher frequency, and it becomes harder to read the additional information. Japanese Patent Laid-Open No. 2009-129222 discloses a method of reading a two-dimensional code for which curve distortion is large.
Japanese Patent Laid-Open No. 2009-129222 determines the existence/absence of curve distortion due to whether a line connecting at least three function cells arranged on a straight line in a two-dimensional code is within a predetermined range of a straight line, and decoding the two-dimensional code (correcting the image) in accordance with a result of the determining.
However, in the case of controlling reading of additional information by determining this kind of image distortion, there are still problems as follows.
In other words, as shown in FIG. 1C, for example, if a capturing angle between an imaging direction 106 for a camera-equipped mobile terminal 105 and an imaged surface 107 is restrained within a predetermined range with respect to what is perpendicular, it is possible to correct the captured image. In contrast to this, as with the relation between the imaging direction 106 and an imaged surface 108, if the capturing angle exceeds a predetermined range, an image of the imaged surface 108 becomes completely unreadable, so that it is not possible to correct the captured image, and as a result, it is not possible to read the additional information. Here, to return the capturing angle between the imaging direction 106 and the imaged surface 108 to what is perpendicular, it is possible to push open the gutter portion 103, but there is a possibility of destroying the photo book by strongly pushing it open.